1. Field of the Invention
The present invention relates to an ink jet head of an ink jet printer including a piezoelectric actuator for applying pressure to ink so as to eject ink droplets onto a recording medium.
2. Related Art
There have been provided thermal type ink jet heads and piezoelectric type ink jet heads, both eject ink droplets from nozzles to form an ink image on a recording medium. The thermal type heads eject ink droplets by an electric heater that generates air bubbles in ink. Because the thermal type heads are produced by a photolithography technology, the nozzle pitch can be as small as less than 100 xcexcm. However, the driving frequency at continuous ink ejection is limited to 10 kHz or less. Also, the life of the heads is as short as several hundred million times of ejection.
On the other hand, the piezoelectric type heads include piezoelectric elements and vibration plates provided in the pressure chamber. When the piezoelectric element deforms, the vibration plate vibrates and applies pressure to ink in a pressure chamber, and so an ink droplet is ejected. Because the deforming amount of the piezoelectric element is relatively small, the vibration plate needs to have a large surface area. This increases a nozzle pitch to about 100 xcexcm. However, because the driving frequency depends on the shape of the piezoelectric elements, the driving frequency can be achieved higher than that of the thermal type heads, and so the piezoelectric type heads are well suited for high speed printing. Also, the piezoelectric heads can be used to eject any type of ink in contrast to the thermal type heads.
An example of the piezoelectric type ink jet head will be described while referring to FIG. 2.
As shown in FIG. 2, a head includes a piezoelectric element 14 fixed to a housing 19, a ceramic member 16 attached to a side of the piezoelectric element 14, and a chamber plate 4 formed with a plurality of pressure chambers 3 (only one is shown in FIG. 2) by etching a stainless material, and an orifice plate 2 formed with a plurality of orifices 1. The piezoelectric element 14 has a vibration surface confronting an ink inlet opening of the orifice 1 via the pressure chamber 3. The orifice 1 and the pressure chamber 3 together define a nozzle. A printer having this type of head is disclosed in Japanese Patent-Application Publication No. HEI-1-115638, for example.
In this type of head, for forming an ink image on a recording medium, ink droplets are continuously ejected from nozzles. However, when the driving frequency is high or when the ejection is performed over a long period of time, there is a possibility that some of the nozzles become defective. In order to prevent such defective nozzle, Japanese Patent-Application Publication No. HEI-9-300613 proposes to drive the piezoelectric elements by a driving signal having a pulse width in a range from 60% to 100% of Helmholtz resonant frequency.
However, because the etching technique is insufficiently precise, the pressure chambers formed in the chamber plate will have uneven shapes and volumes, and the uneven volumes vary the Helmholtz resonant frequencies by relatively large value among the plurality of nozzles. This makes difficult to determine an appropriate pulse width.
It is an object of the present invention to overcome the above problems and also to provide a drive method for an ink jet head to perform ink ejection in a stabled manner over a long period of time.
In order to achieve the above and other objectives, there is provided a drive method for an ink jet head including an ink chamber defined by a diaphragm and connected to an ink tank storing ink, an orifice plate formed with at least one line of plurality of orifices therein, a piezoelectric element which is attached to the diaphragm and deforms when a drive pulse is applied to the piezoelectric element, thereby varying pressure in the ink chamber, an ink channel for supplying ink to the ink chamber, and a housing formed with a common ink channel in fluid communication with the ink channel, the driving method comprising the steps of determining a pulse width, with which an ink speed changes by a small amount when a head difference between a vertical position of the orifice and a vertical position of the head of the ink in the ink tank is changed, and applying a drive signal having the determined pulse width to the piezoelectric actuator.